Wireless communication systems, i.e. systems that provide communication services to wireless communication devices such as mobile phones, smartphones (often denoted by UE that is short for user equipment) as well as machine-type communication (MTC) devices, have evolved during the last decade into systems that must utilize the radio spectrum and other system resources in the most efficient manner possible. A reason for this is the ever increasing demand for high speed data communication capabilities in terms of, e.g., bitrate and to provide these capabilities at any given time, at any geographical location and also in scenarios where the wireless communication device is moving at a high speed, e.g., on board a high speed train.
To meet this demand, within the third generation partnership project (3GPP) work is being done regarding possible enhancements to radio resource management, RRM, performance in high speed train (HST) environments. The justification is that there are railways such as Japan Tohoku Shinkansen (running at 320 km/h), German ICE (330 km/h), AGV Italo (400 km/h), and Shanghai Maglev (430 km/h) at which vehicles travel at greater speed than 300 km/h and where there is demand for using mobile services.
With increased deployment of high speed train lines, increased number of UE users, and increased usage of bandwidth per user, dominating network operators are requesting improved UE performance and support for speeds exceeding 300 km/h. Future high speed trains are expected to travel at speeds above 500 km/h, e.g. the Superconducting Magnetic Levitation train (SCMaglev) to be deployed in Japan, where train sets have already in April 2015 reached more than 600 km/h in speed tests.
For the development of the fifth generation of mobile telecommunication technology (5G), the International Telecommunication Union (ITU) has defined a set of requirements, IMT-2020, which includes the support of UE speeds of above 500 km/h with respect to mobility and data communication.
Apart from the relatively shortened time for detecting suitable neighbor cells for handover or cell reselection, high speed movement of the UE may also lead to significant Doppler shifts of the received radio signals. Such a Doppler shift forces the UE to increase its demodulation frequency when moving towards a cell (i.e. moving towards an antenna that defines a radio lobe of the cell), and decrease demodulation frequency when moving away from a cell, in order to maintain an acceptable receiver performance.
The Doppler shift can be expressed as:
      Δ    ⁢                  ⁢    f    =            v      c        ⁢          f      c      
where c is the speed of light, v is the relative velocity of the UE towards the transmitting antenna and fc is the transmitted frequency.
The magnitude of the Doppler shift depends on the relative velocity of the UE towards the transmitting antenna in a cell. Consequently, with transceivers located close to a path along which an UE is moving, i.e., a small angle between the trajectory of the UE and the line between the UE and the transmitting antenna, a substantial part of the UE velocity will transfer into a Doppler shift. Moreover there will be an abrupt change of sign of the Doppler shift when the UE passes the transmitting antenna and the smaller the angle, the more abrupt is the change from positive to negative Doppler shift.
Each radio propagation path may have its own Doppler shift, depending on how the radio waves travel between the transmitting antenna and the UE. In case of line-of-sight there is one dominant path from each transmitter, whereas in e.g. urban areas there is generally scatter (reflections) due to buildings to which the UE has a relative velocity, giving rise to multiple paths for the signal to propagate to the UE, each with a different Doppler shift. Since the received signal (in general) is the superposition of those paths, it gives rise to Doppler spread which degrades radio receiver performance by smearing out the signal in the frequency domain hence causing inter-carrier interference.
High-speed railway track sections are generally using dedicated platforms often elevated above the landscape or city beneath. Hence, there are few objects that can cause a significant Doppler spread; with cell sites located along the track line-of-sight will be dominating at least between the cell site and the train. Moreover, in built-up areas as well as when a train is approaching or passing stations the speed is generally restricted of concern for public safety and disturbing noise, and as a consequence the Doppler shift becomes small.
Needless to say, there are a number of challenges related to radio receiver control in relation to high-speed train scenarios in prior art. For example, with reference to FIG. 1, an UE 101 traveling on a train 103 at high speed von a railway track 104 along a path and being at a position between two antenna nodes 105, 107, e.g. remote radio heads (RRH), located along the railway track 104 as indicated in FIG. 1, will receive the same signal from at least two directions (i.e. a bidirectional scenario) with opposite Doppler shifts. Such signals are illustrated by line-of-sight (LOS) signals 111, 112 and signals 112 and 122 have other Doppler shifts having been reflected by a structure 110.
This gives rise to a Doppler spread also in the line-of-sight case, due to one path with maximum positive Doppler Shift and one path with maximum negative Doppler Shift. These very large Doppler shifts degrade the receiver performance when signals from both antenna nodes 105, 107 are received with about the same strength. In such a bidirectional deployment between two antenna nodes, the UE receives two signals with very different Doppler frequency shifts, and when the UE is closer to one of the antenna nodes, the signals that travel different signal paths will be received at different times. One of the paths will be stronger than the other one based on the geometry of the situation. Therefore the UE will see two paths with different power due to the geometry, different time due to the different propagation conditions and different frequency due to the sign of the speed relative to the two antenna nodes.
The UE 101 receiver tracks the received frequency with its automatic frequency control (AFC) loop based on its received signals. However, with receivers developed for low speed channels the AFC will not be stable during a period when the UE is between two antenna nodes. Other prior art receivers are discussed in the 3GPP technical report from the Release-13 Study Item on high speed trains, 36.878, v13.0.0 and in the contribution [R4-157700] to 3GPP Radio Access Network workgroup 4 (RAN 4). These radio receivers comprise AFC algorithms that are much more complex than low speed AFC algorithms and they also utilize a special channel estimation for demodulation. A drawback with such prior art receivers and AFC algorithms is, in addition to the fact that the AFC algorithms are very complex, that the AFC algorithms are specifically proposed for the HST bidirectional deployment and they are suitable for use only in a HST environment. The behavior of the AFC algorithms in other environments than in HST environments will with high probability be worse than low-speed algorithms.